Generally, rotary atomizing head type coating machines have been and are in wide use for coating vehicle bodies or similar coating objects. A rotary atomizing head type coating machine is largely constituted by a cover of a tubular shape, an air motor which is housed in the tubular cover, a hollow rotational shaft which is passed axially through and rotated by the air motor, a feed tube which is extended axially and internally of the hollow rotational shaft, and a rotary atomizing head which is mounted on the rotational shaft and put in high speed rotation to spray a paint which is supplied from the feed tube.
As for rotary atomizing head type coating machines, there have been known in the art the so-called direct charging type electrostatic coating machines which are arranged, for example, to apply a high voltage to the rotational shaft for electrically charging paint which flows through the feed tube (e.g., as disclosed in Japanese Laid-Open Patent Publication No. H2-237667, H6-269701 and H8-150352).
In the case of a rotary atomizing head type coating machine of this sort, the rotary atomizing head is formed of an insulating synthetic resin material, and a semi-conductive film layer is formed at and on paint releasing edges of the rotary atomizing head. A high voltage is applied to the semi-conductive film layer through a semi-conductive film, an electrode or the like which is provided in the proximity of the rotary atomizing head. As a result, corona discharge occurs at the fore end of the semi-conductive film layer of the rotary atomizing head, and aeroions are generated by aeroionization under the influence of the corona discharge. Therefore, the paint particles which are sprayed from the paint releasing edges of the rotary atomizing head are adsorbed on aeroions, which are generated in a corona discharge zone, to form charged paint particles. Consequently, charged paint particles which are sprayed from the rotary atomizing head are urged to fly along an electrostatic field toward and deposit on a coating object which is held at the earth potential.
Further, in the case of the prior art as described above, having the rotary atomizing head is formed of an insulating synthetic resin material, it becomes possible to lower the static capacity of the rotary atomizing head itself to a significant degree as compared with rotary atomizing heads which are formed of a metallic material. Therefore, even if a coating object comes to an abnormally close proximity to the rotary atomizing head, there is no possibility of accumulated charges on the rotary atomizing head instantly dischanging toward the coating object.
The prior art rotary atomizing head type coating machines according to the above-mentioned prior art employ a rotary atomizing head which is formed of an insulating synthetic resin material. Accordingly, when a coating object to be coated comes to an abnormally close proximity to the rotary atomizing head, there is little possibility that spark discharges generally referred to as "streamers" or "sparks" are induced solely by accumulated charges on the part of the rotary atomizing head. However, in the case of the above-mentioned prior art, the coating machine has metallic parts such as air motor, which have a floating capacitance and therefore can store electric charges therein. If electric charges are stored by the floating capacitance of the air motor or other metallic parts, spark discharges take place as a result of instant discharge of the stored electric charges to the coating object.
Therefore, it becomes necessary to prevent direct discharges from a metallic component such as air motor, for holding discharge energy below an ignition level. In this connection, discharge energy E can be expressed by Equation 1 below, wherein C is the electrostatic capacitance of a part holding electric charges to be discharged, and V is the voltage across the discharging part and a coating object. EQU E=1/2CV.sup.2 [Equation 1]
Accordingly, in case a rotary atomizing head is formed of an insulating material, the energy of discharges from the rotary atomizing head itself can be suppressed to a low level because its electrostatic capacitance is small. However, in that case it is difficult to suppress discharges from an air motor which is located in the vicinity of the rotary atomizing head and which has a large electrostatic capacitance.
As a countermeasure for preventing discharges from the air motor, it is conceivable to increase the discharge distance between the air motor and a coating object by using a rotational shaft which is formed of an insulating material and relatively large in length. However, in the case of a rotational shaft of an increased length, it is very likely that fluttering occurs to the rotational shaft, thereby impairing mechanical stability of the air motor and shortening its service life to a considerable degree.
Discharges from the air motor can also be prevented, for example, by inserting a high resistance between the air motor and a high voltage generator in such a way as to lower the voltage to be applied to the air motor. However, a reduction in application voltage to the air motor will lead to a reduction in paint particle charging rate and as a result to a reduction in paint deposition efficiency.
Further, there has been a method of providing an anti-spark control circuit in a high voltage generator to prevent such spark discharges as would lead to ignition (e.g., as disclosed in Japanese Laid-Open Patent Publication No. H6-269701). However, a spark preventing method using a control circuit of this sort has a problem that it is not essentially useful for prevention of ignition, as clearly stipulated in Regulations by National Fire Preventing Association of the United States.